0513 — Create-ahead ships with the 95% backstop mark, and destroyed topics evict their claim entry

Context. [0512] deferred the second mark to “only if the drop warn ever shows up in practice” and left createAheadAttempted entries live forever (bounded by distinct topics, stale after a destroy). Building it, both were pulled forward: the second mark is cheap (one more integer compare per append), and the eviction is one Delete at a spot the producer already learns the topic is gone.

Decision.

• Both marks ship now: 80% and 95% of each partition. The claim sequence becomes partition*2 for the 80% mark and partition*2 + 1 for the 95%, keeping one monotonic per-topic CAS value while letting the 95% attempt win a fresh claim after the 80% one already advanced it. The 95% mark is the in-process backstop for an 80% attempt that failed and dropped.
• A create-ahead goroutine whose failure is undefined_table (the topic’s parent table is gone — destroyed while the goroutine was in flight) deletes the topic’s createAheadAttempted entry instead of warning, so the map stays bounded by live topics.

Consequences. Amends [0512]‘s second-mark deferral; everything else there stands. A partition now gets up to two proactive attempts before the boundary heal; the drop warn now signals both marks failing, a stronger signal than before. Eviction only fires if a mark triggers after the destroy — an idle topic’s entry lingers harmlessly until then.